We provide a high-resolution image of the Ivrea Geophysical Body (IGB) in the Western Alps with new gravity data and 3-D density modelling, integrated with surface geological observations and laboratory analyses of rock properties. The IGB is a sliver of Adriatic lower lithosphere that is located at shallow depths along the inner arc of the Western Alps, and associated with dense rocks that are exposed in the Ivrea-Verbano Zone (IVZ). The IGB is known for its high seismic velocity anomaly at shallow crustal depths and a pronounced positive gravity anomaly. Here, we investigate the IGB at a finer spatial scale, merging geophysical and geological observations. We compile existing gravity data and we add 207 new relative gravity measurements, approaching an optimal spatial coverage of 1 data point per 4–9 km2 across the IVZ. A compilation of tectonic maps and rock laboratory analyses together with a mineral properties database is used to produce a novel surface rock-density map of the IVZ. The density map is incorporated into the gravity anomaly computation routine, from which we defined the Niggli gravity anomaly. This accounts for Bouguer Plate and terrain correction, both considering the in situ surface rock densities, deviating from the 2670 kg m–3 value commonly used in such computations. We then develop a 3-D single-interface crustal density model, which represents the density distribution of the IGB, including the above Niggli-correction. We retrieve an optimal fit to the observations by using a 400 kg m–3 density contrast across the model interface, which reaches as shallow as 1 km depth below sea level. The model sensitivity tests suggest that the ∼300–500 kg m–3 density contrast range is still plausible, and consequently locates the shallowest parts of the interface at 0 km and at 2 km depth below sea level, for the lowest and the highest density contrast, respectively. The former model requires a sharp density discontinuity, the latter may feature a vertical transition of densities on the order of few kilometres. Compared with previous studies, the model geometry reaches shallower depths and suggests that the width of the anomaly is larger, ∼20 km in west–east direction and steeply E–SE dipping. Regarding the possible rock types composing the IGB, both regional geology and standard background crustal structure considerations are taken into account. These exclude both felsic rocks and high-pressure metamorphic rocks as suitable candidates, and point towards ultramafic or mantle peridotite type rocks composing the bulk of the IGB.

中文翻译：

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Ivrea 地球物理体（西阿尔卑斯山）的新重力数据和 3-D 密度模型约束

我们提供了西阿尔卑斯山伊夫雷亚地球物理体 (IGB) 的高分辨率图像，其中包含新的重力数据和 3-D 密度建模，并结合了地表地质观测和岩石特性的实验室分析。IGB 是亚得里亚海下部岩石圈的一小部分，位于西阿尔卑斯山内弧的浅层，与暴露在 Ivrea-Verbano 带 (IVZ) 中的致密岩石有关。IGB以其浅地壳深度的高地震速度异常和明显的正重力异常而闻名。在这里，我们在更精细的空间尺度上调查 IGB，合并地球物理和地质观测。我们汇编了现有的重力数据，并添加了 207 个新的相对重力测量值，接近整个 IVZ 每 4-9 平方公里 1 个数据点的最佳空间覆盖范围。使用构造图和岩石实验室分析的汇编以及矿物特性数据库来生成新的 IVZ 地表岩石密度图。密度图被纳入重力异常计算程序，我们从中定义了尼格利重力异常。这说明了布格板块和地形校正，两者都考虑了原位地表岩石密度，偏离了此类计算中常用的 2670 kg m-3 值。然后我们开发了一个 3-D 单界面地壳密度模型，它代表了 IGB 的密度分布，包括上面的 Niggli 校正。我们通过在模型界面上使用 400 kg m-3 的密度对比来检索观测值的最佳拟合，该界面深度可达海平面以下 1 公里。模型敏感性测试表明~300-500 kg m-3 密度对比范围仍然是合理的，因此将界面的最浅部分定位在海平面以下 0 km 和 2 km 深度处，对于最低和最高密度对比，分别。前一个模型需要明显的密度不连续性，后者可能以几公里的密度垂直过渡为特征。与之前的研究相比，模型几何形状达到更浅的深度，表明异常的宽度更大，东西向约 20 公里，E-SE 倾斜陡峭。关于构成 IGB 的可能岩石类型，同时考虑了区域地质和标准背景地壳结构因素。这些排除了长英质岩和高压变质岩作为合适的候选者，